I try to Install W7 on a USB harddrive.
Steps 1 to 7 from your guide work perfectly.
After selecting the main partition from my device, when I press Next, nothing happens. In fact WinToUSB is closed, without any visible message.
My Iso is Windows 7 Ultimate French edition, downloaded from Microsoft.
My harddrive is empty, with NTFS fresh formatted active partition. Connected through a USB3.0 port. (I understood to use W7 I will then have to boot from USB2, but I presume the install can be done through USB3)

Any Idea about the root cause?

Attached a copy of my User.Log :
UserLog.pdf (Size: 116.49 KB / Downloads: 2)
There is also a crash.dmp file, but I didn't find how to include it here